It's bad science. They took plate designed for mounted combat, put someone in it without fitting it to them and tried to see how far people can hike like that. Footsoldiers who wore plate would wear some kind of half plate that left parts of the arm and most of the legs unarmored in order to cut down weight on moving limbs (it takes more energy to move while wearing boots on your feet than on your back). A cursory look at the kinds of armor people actually wore explains away their results.

I wrote 'every time': this implies horses got killed several time. Squires were heavily armed too, not as heavy as their lords though. Dismounted knights fighting as heavy infantry also occurred with a certain regularity. Sure, the main role for knights is on horseback, but this wasn't always te case (Battle of the Standard, Scottish Wars, Hundred Years' War (e.g.: Crécy), Brémule, and in the Holy Land e.g. Richard Lionheart at Jaffa)

Remember that armor evolved over the years., The armor worn by Richard Lionheart at Jaffa was far different than that worn by a typical knight at Crecy.

I'm not a big fan of the study. I read the whole report and some articles. One of the best quotes was something like "nobody does anything in war for no reason" or something like that, the implication being that there must be benefits to wearing armor.

I question the bit about it taking less energy to carry armor on the back than to wear it. I can run in my armor, but walking with it in a pack is more like a controlled stumble. When I try to launch myself forward with a running step while carrying my armor in a backpack, the weight brings me back to earth before I can cover any significant ground. When the armor is distributed across my body and limbs that doesn't happen. I suppose it takes more effort to run in armor than not in armor, but I can do it. Maybe it does take more "energy," but I am certainly more "encumbered" when carrying the armor in a pack.

The biggest reason that I carry my armor in a pack to and from fighter practice is because of the awkwardness of wearing armor while walking through the city ('scuze me *clank* sorry *ow* whoops *watch where you're going buddy* pardon me *clatter* coming through...). The main reason I take it off when I am done is because I fight to complete muscle and/or cardio and/or heat failure, and I need to unload everything to cool down and recover.

And one thing that always irks me about lots of people who talk about anything in the "middle ages" is their lack of chronal specificity. The 900's and the 1300's may both be part of the "middle ages," but so much was different between those two times. The arms and armor of the medieval knight changed so drastically between the rise of medieval knighthood ca. 800 AD and the end of the medieval/renaissance period ca. 1650, that there is a whole genre of history books about it.